1. Field of the Invention
The present invention relates to data storage libraries housing multiple tapes or other data storage cartridges. More particularly, the invention concerns a data storage library that utilizes library-local features to regulate access to shared read/write drives among multiple hosts, thereby avoiding arbitrating host software.
2. Description of the Related Art
A small computer system interface (xe2x80x9cSCSIxe2x80x9d) medium changer is a data storage device including storage slots, drives, and input/output (I/O) slots. SCSI medium changers also include robotic mechanisms that move data cartridges among other SCSI medium changer components. One example of a SCSI medium changer device is an IBM model 3575 tape library.
In many applications, it is desirable to share some or all components of a SCSI medium changer among multiple host applications. Sharing is especially desirable for tape libraries, since they are often used for the limited purpose of backing up data, and otherwise lie dormant during long periods when the tape library could be working for other hosts. Thus, especially for large tape libraries, it is desirable to have several host applications take turns using the library and its tape drives.
One problem with achieving this level of sharing is the need to arbitrate access among the different hosts. Namely, the sharing host applications must have some mechanism to ensure that one host does not access, or worse yet, update the contents of another host""s cartridges. One popular solution is to install intervening software between the library and its hosts to arbitrate access to the library cartridges and tape drives. This software is known as xe2x80x9cmiddleware,xe2x80x9d and numerous examples are commercially available. Nonetheless, there are certain drawbacks. For instance, additional expense is required to purchase and maintain the middleware programs. Installation of the middleware requires the user to sacrifice processing capability of existing host hardware, or invest in additional processing hardware to run the middleware.
In contrast with middleware, a different approach is to logically partition storage slots and tape drives into distinct logical SCSI medium changer devices. This approach is implemented in the IBM model 3575 tape library, for example. The partitioning approach establishes xe2x80x9clogicalxe2x80x9d partitions, each including an assortment of slots and one or more tape drives. Each partition is exclusively associated with one host. Thus, this approach shares the library resources by exclusively associating specific library resources to particular hosts.
Although this approach has certain advantages, engineers at International Business Machines Corp. (xe2x80x9cIBMxe2x80x9d) are continually seeking to improve the performance and efficiency of such systems. In this respect, one area of continual focus concerns the possibility of greater efficiency and device utilization through more efficient sharing schemes. In this respect, the present inventors have recognized that the partition approach still does not sufficiently share the read/write drives, one of the most expensive components in a tape library. Namely, one drive may be extremely busy processing requests of its corresponding host application, while other drives are inactive. Nonetheless, the other drives cannot be recruited to satisfy the busy host""s requests due to the lines of partition.
Consequently, from the standpoint of efficiency, known library sharing schemes may not be completely adequate for some applications due to certain unsolved problems.
Broadly, the present invention concerns a data storage library that utilizes library-local features to regulate access to shared read/write drives among multiple hosts, thereby avoiding arbitrating host software. The library includes multiple data storage media, multiple data storage media slots, multiple read/write drives, and a library controller. The storage slots are originally partitioned into multiple xe2x80x9clogical libraries,xe2x80x9d with one or more hosts having access rights to the slots of each logical library. The controller will prevent a requesting host from removing a cartridge from a source storage slot unless the requesting host has access rights to the logical library of the source storage slot. In contrast to the partitioned storage slots, empty read/write drives are normally shared among all hosts. However, the controller will prevent a requesting host from unloading a cartridge from a read/write drive unless the requesting host has access rights to the xe2x80x9coriginatingxe2x80x9d logical library containing the storage slot from where the cartridge was loaded into the drive.
Accordingly, one embodiment of this invention concerns a method to regulate access to shared read/write drives among multiple hosts, thereby avoiding arbitrating host software. In another embodiment, the invention may be implemented to provide an apparatus, such as a data storage library, including structure to regulate access to shared read/write drives among multiple hosts, and thus avoid arbitrating host software. In still another embodiment, the invention may be implemented to provide a signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital data processing apparatus to perform operations as described herein. Another embodiment concerns logic circuitry having multiple interconnected electrically conductive elements configured to perform such operations.
The invention affords its users with a number of distinct advantages. Importantly, the present invention efficiently utilizes expensive library hardware resources by sharing empty read/write drives among multiple hosts. Accordingly, the present invention treats read/write drives as serially reusable resources, rather than exclusively assigning each drive to one or more hosts. Since this sharing scheme is implemented by the library controller, the invention advantageously utilizes library-local components to arbitrate host access to shared read/write drives. Thus, the invention avoids the need for expensive middleware programs to arbitrate host access to shared read/write drives. Similarly, the invention avoids sacrificing processing capability of existing host hardware that would be caused by running host-based arbitrating software. The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention.